1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method which, during a process of forming an image, correct a bend of a scan line and control the amount of toner applied to a print medium.
2. Description of the Related Art
To achieve a faster image forming speed in an electro photo graphic color image forming apparatus, a growing number of tandem type color image forming apparatuses have come into use in recent years. Such apparatuses have the same number of developers and photosensitive drums as that of colorants and transfer images of different colors successively onto an image carrying belt or print medium. This tandem type color image forming apparatus is already known to have problems contributing to registration shifts and a variety of measures has been proposed to overcome each of the problems.
One of the problems is an unevenness of a lens of a deflection scanning device and a deviation of a mounting position of the lens, and a positional deviation of the deflection scanning device when mounted in a color image forming apparatus body. In such cases, a scan line tilts or bends and the degree of tilting or bending varies from one color to another, resulting in registration shifts among images of different colors.
To deal with this registration shift problem, a measure has been proposed (e.g., Japanese Patent Laid-Open No. 2004-170755) which involves measuring inclination and bend of the scan line by using an optical sensor, correcting bit map image data to cancel or offset measuring inclination and bend of the scan line, and forming the corrected image. This method can correct registration shifts at low cost because mechanical adjustment members and an adjusting step during assembly are obviated by the electrical correction of the image data. This electrical correction of registration shifts is done either in units of one pixel or in units of less than one pixel. In the case of the one-pixel correction, pixels are offset one pixel at a time in a subscan direction according to the amount of correction of inclination and bend, as shown in FIG. 6A-6C. That is, a line is switched to the adjoining line. In the case of the less-than-one-pixel correction, a grayscale value of bit map image data is adjusted using adjacent pixels in the subscan direction, as shown in FIG. 7A-7E. By executing the less-than-one-pixel correction, unnatural steps formed at boundaries offset by the one-pixel correction are eliminated thus smoothing the image (this process is hereinafter referred to as blending).
A heat-pressure fixing device to fix a toner image with heat and pressure, including devices of a heat roller type and a film heating type, is generally used as a fixing means. In an image forming apparatus with such a fixing device, a phenomenon may occur in which a part of the toner of an unfixed toner image on a print medium carried to the fixing device is blown upstream in the print medium transport direction immediately before a fixing nip portion. The blown toner shifts about 0.1-1 mm upstream in the print medium transport direction, making the image look as if it has trailing tails (ends). This is refereed to as a trailing with whisker-like image impairments (hereinafter referred to as a trailing phenomenon). When an image of lines extending in the main scan direction is printed, a problem would occur in which toner applied to the print medium 11 at the rear edge of lines 81 in the subscan direction is scattered, disturbing the printed image, as shown in FIG. 8.
This problem is known to occur under a normal office environments, particularly at high humidity. This is shown in FIG. 9. Water in the print medium 11 is explosively vaporized by a rapid temperature increase and pressing forces of upper and lower rollers (fixing roller 32 and pressure roller 33), with the vapor escaping from a relatively weakly bound rear edge of the deposited toner, scattering the toner in the process. This phenomenon is empirically known to be likely to occur when the width of toner lines in the subscan direction is about 100-1000 μm.
To tackle the above problem, a method (Japanese Patent Laid-Open No. 2000-175029 for example) has been proposed which involves selecting and extracting an image pattern, thinning an image area at a predetermined rate by using the extracted image pattern, and changing an image data thinning method according to an area of the image pattern.
However, when the above electrical registration shift correction and the trailing phenomenon prevention operation are to be executed at the same time, the following problem arises.
Executing the trailing phenomenon prevention operation before the subscan direction line shifting to the next line causes execution of a grayscale correction of less than 1 pixel because the trailing phenomenon prevention operation is followed by the subscan direction line shifting and the image smoothing or blending. Since the image correction is performed even after a decision on the trailing phenomenon prevention operation is made, there is a possibility that the trailing phenomenon may occur even where the trailing phenomenon prevention operation is decided to be unnecessary. This is explained referring to FIG. 10A-10F. The trailing phenomenon prevention operation is performed on an original bit map image. Two pixels 1001, 1002 hit the pattern matching in the trailing phenomenon prevention operation and are eliminated. This is followed by the subscan direction line shifting and the blending operation. Since the decision on blending is based on various image patterns, the amount of toner applied to the print medium changes depending on the decision made. Thus, prior to the subscan direction line shifting and blending operation, there is a possibility that even where the trailing phenomenon prevention operation is decided to be unnecessary, an image printed on a print medium may eventually have a trailing phenomenon. In that case, as shown in FIG. 11, pixels 1003 and 1004 should have also been removed in addition to the pixels 1001, 1002. As described above, the blending operation may cause an erroneous decision to be made as to whether or not to perform the thinning operation.
Another problem is that if the decision on the trailing phenomenon prevention operation is made after the subscan direction line shifting and the blending decision, the fact that the image is changed in the subscan direction when the image pattern is chosen prevents a correct pattern from being extracted before and after the line shifting.
This invention is intended to perform the trailing phenomenon prevention operation normally in a color tandem machine capable of electrically correcting registration shifts.